Invertebrate Nervous Systems

Abstract

The nervous systems of invertebrates are smaller and contain fewer neurons than those of vertebrates, yet they generate the
great diversity and complexity of behaviour that has contributed to the incredible success of these animals.

Keywords: brain; invertebrate; neuron; nervous system; behaviour

Figure 1.

Different types of invertebrate nervous systems. (a) A simple nerve net of loosely associated neurons in a sponge connected
at points of contact by synapses (high magnification view). (b) The ladder‐like layout of a flatworm, with two longitudinal
connectives running the length of the body, joined at intervals by commissures. (c) The segmentally organized nervous system
of an annelid, with the longitudinal connectives and ganglia fused along the midline. There is an elongated ganglion for each
segment. (d) Three ganglia located in the segmented thorax of an insect. The ‘metathoracic ganglion’ is really a fused structure
containing neurons of the metathoracic ganglion and the first three abdominal ganglia.

Figure 2.

Cross‐section through an insect ganglion to show longitudinal bundles (tracts) of axons, neuron somata in a layer around the
margin, commissures containing branches of neurons that cross the midline, and the remaining areas of neuropil where almost
all of the fine branches and synapses are found. In a large insect like a locust, a ganglion measures about 1 mm across.

Lacalli TC
(2001)
New perspectives on the evolution of protochordate sensory and locomotory systems, and the origin of brains and heads.
Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences
356 (1414): 1565–1572.